1. Field of the Invention
The present invention relates to a nitride semiconductor Light Emitting Diode (LED) improved in lighting efficiency and a fabrication method thereof. In particular, the present invention relates to a nitride semiconductor LED, which is improved in lighting efficiency by using a reverse bias tunneling junction and/or lateral current spreading, and a fabrication method thereof.
2. Description of the Related Art
In general, nitride semiconductors are used in LEDs for generating blue or green wavelength light. Such nitride semiconductors may be expressed by an equation of AlxInyGa(1-x-y)N,
wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1.
LEDs for generating green light are produced by using GaN semiconductors having a wide band gap of about 3.4 eV. A nitride semiconductor of for example GaN has a wide band gap, which acts as a problem in the formation of electrodes and ohmic contact structures. In more detail, there are problems in that contact resistance increased in a p-electrode region also raises the drive voltage while enhancing the heat generation of a diode. Although various schemes may be proposed as means for forming the ohmic contact structures, actually adoptable means are extremely restrictive because those regions forming ohmic contact structures also function as major light emitting surfaces, thereby bringing a critical requirement of allowing light from the active layer to pass through the major light transmitting surfaces.
As a conventional technique satisfying such requirements, there is proposed U.S. Pat. No. 5,563,422, which is entitled “Gallium Nitride-Based III–V Group Compound Semiconductor Device and Method of Producing the Same” and assigned to Nichia Chemical Industries, Ltd. This document proposes a transparent electrode layer using a Ni/Au double layer, and a structure of a nitride semiconductor LED disclosed in this document is shown in FIG. 1.
As shown in FIG. 1, a conventional nitride semiconductor LED 10 includes an n-doped GaN cladding layer 13, a GaN/InGaN active layer 15 of a Multiple Quantum Well (MQW) structure and a p-doped GaN cladding layer 17 formed in their order on a sapphire (Al2O3) substrate 11, in which the p-doped GaN cladding layer 17 and the GaN/InGaN active layer 15 are removed in part to partially expose the top surface of the n-dope GaN cladding layer 13. The nitride semiconductor LED 10 also includes an n-electrode 19a formed on the n-doped GaN cladding layer 13 and an ohmic contact structure. The ohmic contact structure is constituted of a transparent electrode 18 of Ni/Au formed on the p-doped GaN cladding layer 17 and a p-bonding electrode 19b formed on the transparent electrode 18. The transparent electrode 18 is provided to improve contact resistance while ensuring transparency, and may be obtained via deposition of a Ni/Au double layer and subsequent heat treatment.
However, according to the afore-described technique, the transparent electrode attached on the p-doped semiconductor layer causes relatively poor ohmic contact properties thereby increasing drive voltage. It is also difficult to improve brightness since only relatively low transparency metals such as Ni/Au are attached on the p-doped semiconductor layer owing to weak bonding force of the p-doped semiconductor layer with respect to metal. Furthermore, the n-doped GaN cladding layer shows relatively bad overvoltage resistance when bonded with the n-electrode.